The invention concerns circuitry detecting what type an optically recorded medium is.
One known type of optically recorded medium is the compact disk (CD). Behind a transparent layer it has a reflecting layer of aluminum with depressions called pits or grooves that represent information stored on the disk. An optical pick-up can read the information on the disk because the reflectivity of the aluminum depends on the pattern of pits. Less light will, due to destructive interference, be reflected from a pit than from an undepressed area, called land.
From the intensity of the light reflected from the disk, accordingly, the pick-up can tell whether the bit it is focused on is a logical ONE or a logical ZERO.
Another type of optically recorded medium is the magneto-optical disk described in the article Magnetooptische Versuche dauern an ["Magneto-optical testing continues"], Funkschau 13 (6/20/1986), 37-41.
In contrast to CD's, magneto-optical disks have no pits. Behind the transparent layer is a magneto-optical layer that information can be stored in and read from. How information is stored on a magneto-optical disk will now be explained.
A laser beam focused on the disk heats the magneto-optical layer to a temperature in the vicinity of its Curie point.
It is usually sufficient to heat the layer to approximately its compensation point, which is below the Curie point. Behind the focal point on the disk is an electromagnet that magnetizes the area heated by the laser beam in one or the other direction. When the laser beam is turned off, the heated area will cool to below the compensation point again, and the magnetic direction established by the elecromagnet will be retained, "freezing in." Individual bits are accordingly stored in domains of different magnetic direction, one corresponding to a logical ONE and the other to a logical ZERO.
The data are read out by exploiting the Kerr effect. The plane of polarization of a linearly polarized beam of light is rotated at a measurable angle when the beam if reflected by a magnetized mirror The plane will be rotated right or left depending on the mirror's direction of magnetization. Since, however, the individual domains on the disk act like magnetized mirrors, the plane of polarization of any beam scanning the disk will be rotated right or left at a measurable angle.
From the rotation of the plane of polarization of the beam of light reflected from the disk, an optical pick-up can determine whether the bit being scanned is a ONE or a ZERO. In contrast to a CD with its pits, a magneto-optical disk can be erased and recorded on again almost as often as desired.
German Patent Application P 3 732 875.1 discloses a magneto-optical recorded medium with information stored both in a magnetic layer and in pits.
German Patent Application P 3 732 874.3 describes a pick-up that can read information from a CD, from a magneto-optical disk, and from the combination CD and magneto-optical disk disclosed in Application 3 732 875.1.
To obtain the information stored in the pits, the light reflected from or shining through the disk is deflected to at least two photodetectors. The data signal is generated by adding the output signals from the two detectors.
The information stored in the magnetic layer is converted to electric signals by separating with a polarizing splitter the light reflected from or shining through the disk into two beams. The beams's planes of polarization are rotated in opposite directions in accordance with the Kerr effect. One beam, with its plane rotated in one direction, is deflected to one photodetector and the other, with its plane rotated in the other direction, to another photodetector. A data signal representing the information stored in the magnetic layer is constructed from the difference between the signals from the two detectors.
The same two detectors can be employed to obtain the information stored in both the pits and the magnetic layer from the light reflected from or shining through the disk. In one case it is the sum of and in the other the difference between their output signals that is constructed.
Since the optical pick-up known from German Patent Application P 3 732 874.3 can be used to read information from a CD, from a magneto-optical disk, and from the combination CD and magneto-optical disk disclosed in Application 3 732 875.1, it will make it easier for the user if the equipment can automatically detect what type of disk is inserted in it.